The art of photoresists is a highly developed technology and many compositions have been described in the literature for use as photoresists. In general these may be divided into two groups according to the manner in which the composition is to be developed after exposure in order to obtain the desired product. The compositions of one group are developed by wet means, usually by application of a solvent which preferentially dissolves either the exposed or the unexposed areas of the resist and which does not appreciably affect the imaged or non-imaged areas, depending on whether the system is positive working or negative working. The other group comprises compositions which are developed entirely by dry means, e.g. by heat or by other forms of energy such as electron beams, lasers or plasmas. The present invention is directed to an improvement of compositions of the second group.
Lewis et al, U.S. Pat. No. 3,925,077 issued Dec. 9, 1975, describes a photoresist composition comprising at least one N-vinyl monomer, an organic iodine compound which serves as a photoinitiator for the polymerization of the N-vinyl monomer and a bleachout dye dissolved or dispersed in a resin matrix. The composition upon exposure to a pattern of suitable radiation followed by the application of heat, produces a relief image. Such compositions are useful in the production of relief phase holograms and can be useful in photochemical milling applications by the use of appropriate physical development techniques. Performance characteristics of this image recording system are described by Zech et al, Appl. Optics, 12, 2822 (1973) who note that the nature of the relief imagery is such that the nonexposed areas are measurably thinner than the exposed areas.
Other photoresist formulations are described in Lewis, U.S. Pat. No. 3,899,338 issued Aug. 12, 1975. This composition comprises at least one polymerizable N-vinyl monomer and at least one organohalogen photoinitiator dispersed in a resin binder. These compositions form a relief image after exposure and application of heat. One of the compositions of matter described in U.S. Pat. No. 3,899,338 was marketed by Horizons Research Incorporated under the trade name "SOLVARIST" U-200. This material was recently described in German Pat. No. 2,726,813 (Dec. 29, 1977) as being capable of undergoing physical development by means of a totally dry process employing a low temperature RF oxygen plasma which is thought to selectively oxidize and volatilize the unexposed regions of the photoresist coating. A similar process utilizing SOLVARIST U-200 has also been reported by C. T. Penn [IEEE Trans. Electron Devices, ED-26, 640 (1979)]. When freshly prepared, the compositions of U.S. Pat. No. 3,899,338 perform extremely well. However, these compositions exhibit an extremely short coated part life prior to exposure which results in a need for increasing exposure time in order to achieve reproducible relief characteristics after heat fixing. One cause of this problem appears to be due to the use of a highly volatile photoinitiator (iodoform) which rapidly diffuses out of the coating matrix at room temperature. The obvious selection of less volatile photoinitiators does indeed result in an extension of coated part stability but at the expense of photoresponse, solution stability and/or relief characteristics after heat fixing.